At least one layer of patterned aluminum is utilized in essentially all silicon integrated circuits. The aluminum pattern is utilized to electrically interconnect various active regions within the integrated circuit. Typically, such aluminum lines are formed by depositing a layer of aluminum at elevated temperatures (250.degree. to 400.degree. C.), forming a polymer resist layer in a desired pattern over the aluminum, and removing through etching the exposed regions of the underlying aluminum to produce the corresponding desired pattern of aluminum electrical connections.
After the deposition of the aluminum layer at elevated temperature the subsequent cooling induces a significant stress in the aluminum layer and ultimately in the lines formed from the layer. This stress results from the difference in coefficients of expansion between the underlying substrate and the aluminum. Typically, the substrate after deposition changes dimension during cooling to a much smaller extent than the aluminum. Thus after cooling, substantial tensile stress is present in the aluminum. The extent of stress depends primarily on the specific composition of the substrate, the design rule employed, the particular device configuration, and the dielectric material used to overcoat the aluminum. However, often with design rules of 1 .mu.m or smaller, the mechanical-stress-gradient is sufficient to cause a movement of the grain boundaries in the aluminum, denominated creep, that results in voids and electrical opens in the aluminum conductors.
Strict design rules also cause additional difficulties. In particular, since narrower aluminum lines are employed at stricter design rules, the same currents as used for devices with less strict design rules yield substantially higher current densities. The relatively high resulting current density with the concomitant current-stress gradient, leads to material motion along aluminum grain boundaries, denominated electromigration, in which opens in the aluminum line are produced. Attempts to avoid electromigration problems have involved, for example, an introduction of a titanium or copper dopant at levels up to 5 atomic percent into the aluminum. These attempts, however, have been considered unsatisfactory because such high concentrations of copper make dry-etch pattern definition more difficult, and lines more susceptible to corrosion. Thus, problems due to both creep and electromigration are present and are likely to become even more serious as design rules become stricter.